Predicate Delegates

A 'predicate delegate' represents a named or anonymous method which has a bool return type i.e. it returns true or false.

Such a method can have any number of parameters and can also be generic.

Predicate delegates are useful for searching or filtering a collection. If they are also generic then they are particularly useful in object oriented design because they eliminate the need to include such functionality in the collection class itself.

A good example of a predicate delegate is the Enumerable.Where extension method which can filter any enunerable collection.

For instance:

using System;

using System.Linq;

class Program

{

static void Main()

{

int[] ia = {1, 3, 4, 5, 6, 2, 9};

// filter out even numbers

var filter = ia.Where(i => i % 2 == 0);

foreach(int j in filter) Console.Write("{0} ", j);

Console.WriteLine();

string[] sa = {"Adam", "Ben", "John", "Alan", "Ahmed", "Sam", "Ravi"};

// filter out names beginning with 'A'

var filter2 = sa.Where(s => s[0] == 'A');

foreach(string t in filter2) Console.Write("{0} ", t);

Console.ReadKey();

}

The output is:

4 6 2

Adam Alan Ahmed

1 Reply

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Vulpes11y ago
A 'predicate delegate' represents a named or anonymous method which has a bool return type i.e. it returns true or false.

Such a method can have any number of parameters and can also be generic.

Predicate delegates are useful for searching or filtering a collection. If they are also generic then they are particularly useful in object oriented design because they eliminate the need to include such functionality in the collection class itself.

A good example of a predicate delegate is the Enumerable.Where extension method which can filter any enunerable collection.

For instance:

using System;
using System.Linq;

class Program
{
static void Main()
{
int[] ia = {1, 3, 4, 5, 6, 2, 9};
// filter out even numbers
var filter = ia.Where(i => i % 2 == 0);
foreach(int j in filter) Console.Write("{0} ", j);
Console.WriteLine();

string[] sa = {"Adam", "Ben", "John", "Alan", "Ahmed", "Sam", "Ravi"};
// filter out names beginning with 'A'
var filter2 = sa.Where(s => s[0] == 'A');
foreach(string t in filter2) Console.Write("{0} ", t);
Console.ReadKey();
}
}

The output is:

4 6 2
Adam Alan Ahmed